In order to reassess the relative importance of genetic and environmental factors in the development of myopia in Chinese schoolchildren, 90 pairs of MZ and 36 pairs of like-sex DZ twins were enrolled for detailed ophthalmological examination about their ocular refractions. Corneal curvatures and axial lengths were also measured. With equivalent settings of the range for concordance, corneal curvatures showed higher concordance rate (84%) than ocular refractions (65%) and axial lenghts (59%) in MZ twins. F-test on the intrapair variances between MZ and DZ twins revealed a significant hereditary role in determining the ocular refraction and its optical components. The degree of genetic determination was expressed by heritability indices, which were derived from intraclass correlation coefficients. The diversity of sample ages and refractions, while making the obtained data difficult to further explore the gene-environment interaction, led to the observation of more intrapair differences with age and myopic progression in MZ twins.

In the Louisville Twin Study, pairs of 3- and 4-year-old twins were provided with standardized competitive or cooperative tasks in a laboratory setting. Some tasks required a pair of twins to share toys; other fostered a more competitive engagement between the twins. Behavioral ratings identified temperament and social components at both ages, and between the two ages, there were transformations in the links between the components. Parental ratings of the twins' temperament at the same ages were moderately correlated with the laboratory observations, but the pattern of the relations changed from one age to the next. The combined sets of measures were subjected to twin analyses for 43 pairs of twins. The results are discussed in terms of the similarity of MZ and DZ pairs for the dimensions of temperament and the transformations of temperament.

The predictive relationship between neonatal behavior and temperament at 6 months was assessed in 62 pairs of twins. The neonatal assessment evaluated irritability, resistance to soothing, reactivity, and activity level across various activities. Temperament at 6 months was appraised by a laboratory assessment, ratings on the Infant Behavior Record, and a questionnaire completed by the mother. Examination for temperament consistency from the neonatal period to 6 months indicated that (a) neonates rated by an examiner as irritable and difficult to soothe were reported by the mother to be more negative, irritable, and slower to adapt at 6 months; (b) neonates rated as more irritable and active were more attentive and responsive during mental testing at 6 months; (c) there was no relationship between the neonatal measures and the laboratory ratings, possibly reflecting methodological problems influenced by maturation and vegetative processes at 6 months. Analyses for patterns of concordance within the various data sets for MZ and DZ twins indicated that there was no pattern of differential concordance favoring MZ twins for the temperament measures at birth or at 6 months. It was concluded that a modest predictive relationship between temperament variables had been demonstrated from birth to 6 months, but no specific genetic effects were detected at these early ages.

In the Louisville Twin Study, laboratory observations of twin infants' temperament at 12, 18, and 24 months were linked with parental ratings from temperament questionnaires. Core dimensions of temperament were extracted by factor analysis applied to each set of measures at each age. The laboratory temperament dimension was recurrently represented by emotional tone, social orientation, attentiveness, and reaction to restraint. The questionnaire temperament dimension was recurrently represented by mood, approach/withdrawal and adaptability. The laboratory and questionnaire dimensions were found to be correlated at each age (convergent correlations: 0.38 to 0.52) and to be stable across ages (stability correlations 0.37 to 0.66). The temperament dimensions were used to demonstrate that temperament profiles were more concordant for identical than for fraternal twin pairs. The results demonstrate the genetic influences on (a) the primary dimensions of temperament and (b) the developmental transformations of temperament.

Hospitalization rates of monozygotic (MZ) and dizygotic (DZ) twin pairs in Finland were compared for schizophrenia, neuroses, and alcoholism. Record-linkage of hospital records and death certificates for the years 1972-1979 was carried out for persons in the Finnish Twin Cohort (16,649 like-sexed twin pairs). The ratio of the number of observed vs that of expected concordant pairs and the ratio of concordance rates between MZ and DZ pairs were greater among males than females, and greater among young (40 years old or less) than among older pairs. The highest difference was found in schizophrenia and the lowest in neuroses. Pairwise concordance rates for schizophrenia (11.0% for MZ and 1.8% for DZ) seem to indicate great environmental influence (high proportion of discordant pairs) with apparent genetic liability (6.1-fold ratio in concordance between MZ and DZ pairs). In neurotic disorders, the difference of pairwise concordance rates between MZ and DZ pairs (6.8% vs 4.0%) was quite low, not strongly supporting a genetic hypothesis. Of the MZ pairs concordant for psychiatric hospitalization, 47% had lived together for their whole life time; of those discordant, 16% lived together. The corresponding figures for DZ pairs were 18% and 15%. The effect of intrapair relationships in disease-concordant pairs should be taken into account when evaluating the effect of genetic and environmental factors in psychiatric disorders.

Scores from a behavioral assessment of 160 stable preterm newborn twins were compared with those obtained from 120 full-term newborn twins. The twins were examined during various activities to obtain representative samples of behavior in the following behavioral categories: irritability, resistance to soothing, reactivity, reinforcement value, and activity level. Within-pair correlations were found in all of the behavioral categories, suggesting a constitutional influence on behavioral patterns during the neonatal period. The preterm infants demonstrated internal consistency of behavior similar to that of the full-term group in the first three categories. However, group differences were found when comparing the preterm with the full-term infants on actual ratings of the items. Behavioral differences between these full-term and preterm twins were comparable to those published for samples of singleton infants.

A flexible method based on maximum likelihood theory is introduced for the analysis of binary response data in twins. The method allows for explanatory variables such as age and sex, is free of the untestable distributional assumption of bivariate normality of liability, and makes more efficient use of the data available. The method is illustrated with preliminary data on drinking status in adult twins. Although there is some bias in the ascertainment of male dizygous twins, the results suggest that monozygous twins are more concordant than dizygous twins for drinking status.

The application of multiway contingency table analysis to the multivariate analysis of concordance ratios in twin studies is developed. The approach is illustrated by data on smoking and alcohol use in Finland and Sweden. This approach can enable the assessment of the effect of other variables on the concordance ratio and thus allow estimates of genetic effects on the trait under study. Hypotheses on relationships between genetic effects and other variables can be tested. After hypothesis testing, model fitting of the best hypothesis can be carried out.

A statistical index is proposed in order to determine twin zygosity based on weight and height measures and a Bayesian test of hypotheses. Some Roman twin data are analyzed on the basis of the Euclidean distance index, which appears to be the best for an easy calculation and for its genetic interpretation based on the formulation of genetic distance.

Twin data on bipolar and unipolar affective disorders are analyzed by multiple threshold models of inheritance. The two illness types are represented in the models on a continuum of genetic–environmental liability in which bipolar illness has a higher liability threshold than unipolar disorder. Autosomal single major locus model provides an acceptable fit to observed concordance rates in monozygotic twins. The multifactorial-polygenic model is rejected.

The formula H′C = 1 − CDZ/CMZ is suggested as a better summary of twin concordance data than the familiar Holzinger concordance formula, HC = (CMZ − CDZ)/ (1 − CDZ). The new formula better estimates degree of genetic determination, G, as calculated from a threshold model, but never exceeds unity, as G sometimes does. For high concordance rates, if CMZ + CDZ > 1, HC, may be more useful than either G or H′C.